1. Field of the Invention
This invention relates to a radio base station capable of detecting the position of a terminal device (a mobile station) during communication. It also relates to said terminal device, and to the transmission power control method employed by said terminal and base station.
2. Description of Related Art
It is well known that in a cellular mobile communication system employing code division multiple access (CDMA), all the users transmit information simultaneously using the same frequency band. As a result, the users' signals mutually interfere. A radio signal received by a base station from the terminal (a mobile station) of a user located near the base station is much stronger than a radio signal received from another user terminal located in the vicinity of the radio cell boundary. To express this differently, a signal transmitted from a user terminal that is far from a base station undergoes greater attenuation than a signal transmitted from a nearby user terminal. This phenomenon is called the “near-far effect” or the “near-far problem”. Consequently, a user terminal that is far from a base station is screened by a user terminal in the vicinity of the base station. In order to maintain sufficient capacity, all signals must arrive at the base station with the same average power irrespective of the distance from the user to the base station. One solution to this problem is transmission power control that keeps the average received power at the base station constant.
Hence in a CDMA based cellular mobile communication system, high-precision transmission power control is necessary for stable operation of the system as a whole, and in particular for maintaining subscriber capacity.
The physical channels used in a CDMA system can be classified into a perch channel, a common physical channel and a dedicated physical channel. The perch channel is a unidirectional channel from a base station to a mobile station, and is initially seized by the mobile station when its power is turned on. The common physical channel is shared by the plurality of mobile stations that are present within one and the same cell. For example, in the downlink direction from a base station to a mobile station, the common physical channel carries the paging channel for paging mobile stations. This paging channel is transmitted from a plurality of base stations simultaneously and, in order to reduce mobile station power consumption while the mobile station is waiting, has a sleep mode function that uses grouping. After the cell in which a specific mobile station is located has been identified, a variety of control signals are exchanged through control channels provided as an uplink/downlink pair. The dedicated physical channel is used to exchange user data between a base station and a specific mobile station, and to exchange a variety of control information during a call, and this channel has a variable rate function.
CDMA systems generally apply a combination of open loop control and closed loop control to facilitate high-precision transmission power control under conditions of rapid movement. Transmission power control can be divided into uplink (or reverse link) transmission power control and downlink (or forward link) transmission power control. The main purpose of uplink transmission power control is to overcome the near-far problem mentioned above. As opposed to this, the aim of downlink transmission power control is to decrease interference.
A description will first of all be given of uplink transmission power control.
Transmission power control by a mobile station is implemented by an open loop based on measurement of the received power in the perch channel. In other words, a mobile station recognizes the base station transmission power as reported over the perch channel, compares this with the measured received power in the same channel, and thereby estimates the propagation loss. The mobile station also uses the received power of interfering signals at the base station, this having been reported over the perch channel, to determine the transmission power in the common physical channel, which it does in accordance with a prescribed formula. In other words, in open loop control, a mobile station determines its transmission power on the basis of the average received power.
To provide more precise control, in addition to the aforementioned open loop control, closed loop transmission power control is applied in the dedicated physical channel over which user data is transmitted. This closed loop control is based on measurement of the signal-to-interference ratio (SIR), which is the ratio of the power of the desired wave to that of the interfering wave. The SIR is also called the carrier/interference (C/I) ratio.
A base station makes a slot-by-slot comparison of the measured SIR and the target SIR. On the basis of the result of this comparison, the base station uses a transmission power control (TPC) bit inserted in the data signal to command the mobile station to increase or decrease its transmission power. This TPC bit serves to instruct the mobile station to increase or decrease its transmission power, and is also referred to as a control instruction or a transmission power control command. In this sequence of operations, the most reliable way of evaluating transmission quality is in terms of bit error rate (BER) or frame error rate (FER), and hence outer loop control is provided in addition to inner loop control based on SIR. The outer loop control changes the target SIR dynamically in accordance with the results of measurement of BER or FER, these results reflecting the diversity effect achieved by transmitting from a variety of sectors within the base station.
The mobile station determines PDPCH, the transmission power in the dedicated physical channel, by adding Pclosed-loop, the content of the TPC bit sent from the base station, to Popen-loop, the result previously obtained by the open loop control. In other words, PDPCH=Popen-loop+Pclosed-loop.
A description will next be given of downlink transmission power control.
Two methods of downlink transmission power control are known. The first involves controlling base station transmission power on the basis of the position (distance) of a mobile station, and is sometimes called “distance-driven transmission power control”. The second, like the closed loop control described above in connection with uplink transmission power control, involves controlling base station transmission power on the basis of SIR or C/I. This second method is sometimes called “C/I-driven transmission power control”.
First of all, distance-driven transmission power control will be described. If the position (distance) of a mobile station is known, the combined transmission power of the base stations can be minimized by a base station transmitting to a mobile station at the cell boundary at a high transmission power and transmitting to a mobile station in the vicinity of the base station at a low transmission power. Distance-driven transmission power control is particularly suitable for non-shadowing environments, where “shadowing” signifies the slow signal fluctuation that often occurs in outdoor radio environments due to the signal being screened by hills, buildings, etc. This is because attenuation of power is determined only by distance. A pilot signal has to be transmitted from the base station so that mobile stations can measure their distance from the base station.
Next, C/I-driven transmission power control will be described. This method is intended to reduce the C/I ratio (SIR) of each user (i.e. of each mobile station) on an individual basis. Each mobile station has to transmit information relating to its C/I ratio (i.e., a control instruction) to the base station. On the basis of this information, the base station is able to decide whether its transmission power to a specific mobile station should be increased or decreased. In other words, high channel capacity is achieved by performing transmission power control so that the SIR attains a prescribed value. This will now be described in greater detail. A mobile station measures the downlink reception quality and compares this measured reception quality with a control target value. If the measured reception quality is higher than the target, the mobile station transmits to the base station a control instruction causing it to reduce its transmission power. Conversely, if the measured reception quality is lower than the control target, the mobile station transmits to the base station a control instruction causing it to increase its transmission power. The base station increases or decreases its transmission power in accordance with these control instructions.
A conventional transmission power control method will now be described with reference to FIG. 7, which is a block diagram showing the configuration of a conventional mobile communication system. The illustrated transmission power control method corresponds to the aforementioned downlink C/I-driven transmission power control, and the depicted mobile communication system comprises base station BS and mobile station MS. In the example of FIG. 7, the left-hand side is the base station and the right-hand side is the mobile station.
Base station BS comprises receiving antenna 1 for receiving the uplink radio signal transmitted via the uplink from mobile station MS, receiver 2 connected to this antenna 1, received uplink data output terminal 3 connected to receiver 2, downlink transmission power control command extractor 4 connected to receiver 2, downlink transmission power controller 9 connected to this downlink transmission power control command extractor 4, downlink data input terminal 10 to which downlink data is supplied, transmitter 11 connected to this downlink data input terminal 10 and to downlink transmission power controller 9, and, connected to this transmitter 11, transmitting antenna 12 for transmitting the downlink radio signal via the downlink to mobile station MS.
Mobile station MS comprises receiving antenna 13 for receiving the downlink radio signal transmitted via the downlink from base station BS, receiver 14 connected to this receiving antenna 13, demodulator 15 connected to this receiver 14, received downlink demodulated data output terminal 16 connected to demodulator 15, downlink transmission path state estimator 17 connected to receiver 14, downlink transmission power control command generator 18 connected to this downlink transmission path state estimator 17, uplink data input terminal 19 to which uplink data is supplied, mixer 20 connected to this uplink data input terminal 19 and to downlink transmission power control command generator 18, transmitter 21 connected to this mixer 20, and, connected to this transmitter 21, transmitting antenna 22 for transmitting the uplink radio signal via the uplink to base station BS.
In base station BS, when transmitter 11 receives downlink data from downlink data input terminal 10, it spreads this downlink data and transmits the spread signal from transmitting antenna 12 as a downlink radio signal.
In mobile station MS, receiving antenna 13 receives the downlink radio signal transmitted from transmitting antenna 12 of base station BS and sends it to receiver 14. Receiver 14 despreads this received downlink signal and outputs the despread signal to demodulator 15. Demodulator 15 demodulates the despread signal and outputs the demodulated downlink data from downlink demodulated data output terminal 16.
Independently of this, the downlink signal from receiver 14 is supplied to downlink transmission path state estimator 17. Downlink transmission path state estimator 17 estimates the state of the downlink transmission path from the received signal obtained from receiver 14. By way of example, downlink transmission path state estimator 17 makes this estimation by calculating the signal-to-interference+noise ratio (SINR: the ratio of the received power of the desired wave (S) to the sum of the received power of the interfering wave (I) and the noise power (N)). It is of course also feasible to use the aforementioned SIR instead of the SINR. Downlink transmission path state estimator 17 sends, to downlink transmission power control command generator 18, an estimated transmission path signal indicative of the estimated state of the downlink transmission path. On the basis of the estimated state of the downlink transmission path indicated by this estimated transmission path signal, downlink transmission power control command generator 18 generates a downlink transmission power control command for either increasing or decreasing the downlink transmission power of base station BS. This will now be described in greater detail. Downlink transmission power control command generator 18 compares, by way of example, the calculated SINR and the target SINR, and if the calculated SINR is smaller than the target SINR, it generates a downlink transmission power control command instructing base station BS to increase its downlink transmission power. On the other hand, if the calculated SINR is larger than the target SINR, downlink transmission power control command generator 18 generates a downlink transmission power control command instructing base station BS to decrease its downlink transmission power.
The downlink transmission power control command generated by downlink transmission power control command generator 18 is supplied to mixer 20. Mixer 20 mixes this downlink transmission power control command with the uplink data supplied from uplink data input terminal 19 and sends the mixed uplink data to transmitter 21. Transmitter 21 spreads the mixed uplink data and transmits the spread signal from transmitting antenna 22 as the uplink radio signal.
In base station BS, receiving antenna 1 receives the uplink radio signal and feeds it to receiver 2. Receiver 2 despreads and demodulates the received uplink signal. The uplink data thereby obtained is output from received uplink data output terminal 3. In order to extract the downlink transmission power control command from the received uplink signal, the uplink signal (information) is sent from receiver 2 to downlink transmission power control command extractor 4. The downlink transmission power control command extracted by downlink transmission power control command extractor 4 is sent to downlink transmission power controller 9. In response to the downlink transmission power control command, downlink transmission power controller 9 determines the next transmission power by instructing transmitter 11 to increase or decrease its downlink transmission power by a fixed amount. Transmitter 11 then transmits at a downlink transmission power that conforms to the instruction from downlink transmission power controller 9.
The reception quality at mobile station MS is thus kept constant by controlling the downlink transmission power of base station BS in such manner that, from the point of view of mobile station MS, the state of the downlink transmission path remains constant.
Although the example shown in FIG. 7 was used to describe downlink transmission power control, the same configuration could also be made to perform uplink transmission power control by taking the right-hand side of FIG. 7 as the base station and the left-hand side as the mobile station.
FIG. 8 is a block diagram showing a conventional mobile communication system which implements a conventional uplink transmission power control method. In this example, base station BS comprises, instead of downlink transmission power control command extractor 4 and downlink transmission power controller 9, uplink receiving state estimator 23, uplink transmission power control command generator 25 and mixer 26. Mobile station MS comprises, instead of downlink transmission path state estimator 17, downlink transmission power control command generator 18 and mixer 20, uplink transmission power control command extractor 27 and uplink transmission power controller 28.
In base station BS, receiving antenna 1 receives the uplink radio signal and supplies it to receiver 2. Receiver 2 despreads and demodulates this uplink signal. The uplink data thereby obtained is output from received uplink data output terminal 3.
Independently of this, the uplink signal from receiver 2 is supplied to uplink receiving state estimator 23. Uplink receiving state estimator 23 uses the signal from receiver 2 to estimate the uplink receiving state. This uplink receiving state can be estimated from the uplink signal level, or from the SIR of the demodulated data, or from the BER of the demodulated data, or from the FER of the demodulated data, or from the estimated state of the transmission path, etc. Uplink receiving state estimator 23 sends, to uplink transmission power control command generator 25, an estimated receiving state signal indicative of the estimated receiving state. On the basis of the estimated uplink receiving state indicated by this estimated receiving state signal, uplink transmission power control command generator 25 generates an uplink transmission power control command for either increasing or decreasing the uplink transmission power of mobile station MS. This will now be described in greater detail. Uplink transmission power control command generator 25 compares, by way of example, the estimated uplink receiving state with a reference value, and if the estimated uplink receiving state is smaller than the reference value, it generates an uplink transmission power control command instructing mobile station MS to increase its uplink transmission power. On the other hand, if the estimated uplink receiving state is greater than the reference value, uplink transmission power control command generator 25 generates an uplink transmission power control command instructing mobile station MS to decrease its uplink transmission power.
The uplink transmission power control command generated by uplink transmission power control command generator 25 is supplied to mixer 26. Mixer 26 mixes this uplink transmission power control command with the downlink data supplied from downlink data input terminal 10, and sends the mixed downlink data to transmitter 11. Transmitter 11 spreads the mixed downlink data and transmits the spread signal from transmitting antenna 12 as the downlink radio signal.
In mobile station MS, receiving antenna 13 receives the downlink radio signal transmitted from transmitting antenna 12 of base station BS and sends it to receiver 14. Receiver 14 despreads this downlink received signal and sends the despread signal to demodulator 15. Demodulator 15 demodulates the despread signal and outputs the demodulated downlink data from received downlink demodulated data output terminal 16. In order to extract the uplink transmission power control command from the received downlink signal, the downlink signal (information) is sent from receiver 14 to uplink transmission power control command extractor 27. The uplink transmission power control command extracted by this uplink transmission power control command extractor 27 is sent to uplink transmission power controller 28. In response to the uplink transmission power control command, uplink transmission power controller 28 determines the next uplink transmission power by instructing transmitter 21 to increase or decrease its uplink transmission power by a fixed amount. Transmitter 21 then transmits at an uplink transmission power that conforms to the instruction from uplink transmission power controller 28.
The average receiving power at base station BS is thus kept constant by controlling the uplink transmission power of mobile station MS in such manner that the receiving state at base station BS remains constant.
A variety of prior art techniques relating to such a C/I-driven transmission power control method have been proposed. For example, JP 2000-022611 A (hereinafter called the “first prior art reference”) discloses a “transmission power control method and radio communication apparatus” for a system successively changing directivity in accordance with the position of a distant party and the propagation environment, which controls both the transmission directivity gain and the transmission power to increase communication quality. This first prior art reference adopts a configuration whereby the transmission directivity gain is calculated on the basis of reception weights calculated by an adaptive array antenna; transmission weights are controlled on the basis of this calculated transmission directivity gain; and the gain of the transmission power amplifier is controlled in accordance with transmission power control information contained in the received signal.
JP 2000-138633 A (hereinafter called the “second prior art reference”) discloses a “transmission power control method, transmission power control apparatus, mobile station, base station and control station” which, in propagation environments such as multipath and/or when the rate of movement of a mobile station changes, rapidly alters the control target of the transmission power control, thereby keeping channel quality constant and providing the desired channel quality. In this second prior art reference, each time a frame is received from a distant station, if a frame error is detected, the control target (e.g., the signal-to-interference power ratio) is increased by SIRinc, and otherwise the control target is decreased by SIRdec, where SIRdec is the product of the target FER and SIRinc.
JP 2000-165320 A (hereinafter called the “third prior art reference”) discloses a “transmission power control apparatus and transmission power control method” adapted to increase transmission power only when necessary for position detection. In this third prior art reference, if position detection has been performed correctly, a control signal closes a control switch and transmission power control is carried out by the transmission power control circuit on the basis of the received power of the signal obtained by combining direct and delayed waves. On the other hand, if position detection has not been performed correctly, a control signal opens the switch and transmission power control is carried out on the basis of the received power of the direct wave only.
A variety of prior art techniques relating to distance-driven transmission power control have also been proposed. For example, JP 2000-197118 A (hereinafter called the “fourth prior art reference”) discloses a “radio communication apparatus and method for setting specifications of a radio communication apparatus” capable of automatically setting the frequency band, transmission power, etc. in accordance with the specification of the radio waves used in the service region. In this forth prior art reference, a position information detector uses an antenna to receive a radio wave from a GPS satellite and inputs this radio wave to a GPS receiver. The GPS receiver despreads and demodulates the spread-spectrum signal transmitted from the GPS satellite and sends the demodulated signal to a positioning unit. The positioning unit extracts, from the demodulated signal, information relating to the position when the radio communication apparatus is used, and outputs it to the specification controller CPU. This CPU accesses the memory on the basis of the position information sent from the positioning unit, reads from the memory the radio wave specification corresponding to service region, controls the oscillation frequency of the oscillator, and controls power by controlling the amplifier gain.
JP H08-256103 A (hereinafter called the “fifth prior art reference”) discloses a “radio communication system” which, in a radio communication system comprising a base station and a plurality of mobile stations, is very economical and can very easily control the transmission power of the mobile stations in accordance with the relative distance between them and the base station, without causing the frequency utilization efficiency to decrease. This fifth prior art reference also discloses a “radio communication system” which can very easily control the transmission power of the mobile stations in similar manner even if the base station is a semi-fixed station capable of movement. In this fifth prior art reference, each mobile station can comprise: means for utilizing GPS or other position data to detect its own position; means for calculating the relative distance between itself and the base station from its own detected position and from preset information relating to the position of the base station; and control means for controlling its own transmission power in accordance with the calculated relative distance. Further, according to this fifth prior art reference, if the base station is a semi-fixed station capable of moving, then the base station, too, is provided with means for detecting its own position and is adapted to transmit information relating to its own detected position to the mobile stations.
JP H09-121193 A (hereinafter called the “sixth prior art reference”) relates to a “radio communication system, base station and mobile station” for communicating by radio signals, and discloses technical ideas that enable signals to be transmitted at an efficient transmission power regardless of changes in communication time and received electric field. In the context of a radio communication system wherein mobile stations communicate by radio via a base station, this sixth prior art reference is configured so that a mobile station detects its own position and transmits information relating to this position to the base station. The base station holds information relating to base station transmission powers corresponding to mobile positions within the base station's radio area, and determines its own transmission power by referring to the base station transmission power information corresponding to the position information sent from the mobile station. The base station then controls its transmission power on the basis of the transmission power value that is found in this way.
Prior art that combines distance-driven transmission power control and C/I-driven transmission power control has also been proposed. For example, Japanese Patent No. 3109311 (hereinafter called the “seventh prior art reference”) discloses a mobile station radio transceiver, a base station radio transceiver and a mobile communication system which, in a mobile communication system that adopts spread spectrum communication, are capable of controlling the transmission power from a mobile station more accurately than previous systems, without calls being dropped unexpectedly. The base station radio transceiver disclosed in this seventh prior art reference calculates the distance between the base station and the mobile station on the basis of information relating to the latitude and longitude of the mobile station and information relating to the latitude and longitude of the base station. The base station transceiver then performs control wherein a communication protocol is relaxed in accordance with this calculated distance and the quality of the received signal. For example, when a mobile station is located within the service area of the base station, the base station transceiver relaxes the communication protocol, adopted by previous systems, that causes the communication to be terminated. This makes it possible to prevent a call from being dropped when the receiving level temporarily decreases. The mobile station radio transceiver disclosed in this seventh prior art reference controls transmission power on the basis of (i) information relating to the distance to the base station and (ii) the quality of the received signal. This prevents the use of excessive power when transmitting to the base station, which can occur when for example a mobile station has been screened by something, the output amplifier gain has increased due to the receiving level decreasing, and then the mobile station has come out from behind whatever was screening it. The mobile station radio transceiver disclosed in this seventh prior art reference also notifies the user, again, on the basis of (i) information relating to the distance to the base station and (ii) the quality of the received signal, of deterioration in the received signal when the distance to the base station is within a prescribed range and the signal has deteriorated. This lets the user know that the call has not been dropped.
The problems described below are encountered with conventional transmission power control methods of the sort disclosed in the prior art illustrated in FIG. 7 and FIG. 8, and in the seven prior art references outlined above.
The first problem is that changes in transmission power do not keep up with sudden changes in the state of the transmission path (i.e., in the receiving state).
A specific example of this will be described with reference to FIGS. 9a and 9b, based on the mobile communication system shown in FIG. 7. FIG. 9a shows the transmission power of base station BS and FIG. 9b shows the receiving level at mobile station MS, with mobile position plotted along the horizontal axis. FIGS. 9a and 9b illustrate a case where the state of the transmission path suddenly changes from good to bad as a result of the position of mobile station MS. When the state of the transmission path suddenly deteriorates, the receiving level at mobile station MS drops as shown in FIG. 9b, which means that communication quality deteriorates. This can result in a call being dropped.
This shortcoming of conventional transmission power control methods is due to the fact that, whether a C/I-driven or a distance-driven transmission power control method is employed, control is performed on the basis of the present (or past) state of the transmission path (i.e., on the basis of the present or past receiving state). In other words, in a conventional C/I-driven transmission power control method, the present transmission path state (receiving state) is estimated from the calculated present SIR or SINR, and transmission power is controlled on the basis of this estimated present (or past) transmission path state (receiving state). In conventional distance-driven transmission power control methods, the present transmission path state (receiving state) is estimated from the present position of the mobile station, and transmission power is controlled on the basis of this estimated present (or past) transmission path state (receiving state). As a result, it becomes difficult for changes in transmission power to keep up with sudden changes in transmission path state (receiving state).
A second problem is that if the transmission path state (the receiving state) has become so poor that the prescribed communication quality is not obtained even when transmitting at maximum power, the very high transmission power constitutes significant interference to other users.